In ink compositions for writing instruments prepared by using a thermochromic coloring material making use of a developing and fading mechanism of a leuco dye, a pigment prepared by microcapsulating the above colorant has so far been usually used.
Known are, for example, (a) a specific lactone derivative as an electron-donating coloring-type organic compound, (b) an electron-accepting compound which is a developer and (c) a reversible thermochromic microcapsule pigment including a reaction medium reversibly causing an electron donating and accepting reaction by the both components described above in a specific temperature region (refer to, for example, patent document 1), and azaphthalides are listed as the specific colorant (compound) in the above patent document 1.
However, azaphthalides are not still unsatisfactory in a color intensity and a lightfastness, and a microcapsule pigment and an ink for writing instruments which are prepared by using the same involve the problems that the drawn lines are short of an intensity and that the drawn lines are time-dependently faded in a certain case.